1. Field of the Invention
The present invention relates to a temperature compensated voltage supply circuit, in particular to a technique and circuit to develop a temperature compensated voltage from two or more base voltages having different temperature coefficients without using complicated modulation processes and to convert a positive temperature coefficient to a negative temperature coefficient to suit wider applications.
2. Description of Related Arts
In circuit designs, a reference voltage is often an input voltage that is invariant to changes in temperature, which means the temperature coefficient (TC) of the reference voltage has a zero value, where the temperature coefficient (TC) can be defined by the expression:   TC  =                              F          ⁡                      (            T2            )                          -                  F          ⁡                      (            T1            )                                      F        ⁡                  (          T0          )                            T2      -      T1      Where:                F( ) function represents a parameter, which is a voltage in the present example;        T0, T1, and T2 represent different temperature values, and T0 is a reference temperature; and        F(T0), F(T1), and F(T2) represent three voltages having three different temperatures.        
However for certain special requirements, the voltage value is expected to vary in accordance with any temperature variation (in which case the temperature coefficient is not a zero value), and the temperature coefficient needs to be controllable. The most important point is that the output voltages at the reference temperature (T0) must always be the same even with different temperature coefficients. With reference to FIG. 7, the characteristic curves of two different temperature coefficients (TC1, TC2) show that TC1 value eventually exceeds TC2, but their output voltages are all equal to V0 at the reference temperature T0.
This circuit design is commonly found in the liquid crystal display (LCD) devices. Since a LCD device is generally sensitive to temperature, the control voltage has to be precise to match the changes in temperature for the precision operation in an LCD device.
With reference to FIG. 8, a bandgap reference circuit can be used to control the temperature coefficient by limiting the current passing through a diode, represented by the following expression:   I  =                    I        o            ×              e                  V                      KT            q                                =                  I        o            ×              e                              q            ⁢                                                   ⁢            V                    KT                    where the output current Iout can be expressed as:       I    out    =            KT      q        ×                  ln        ⁡                  (          N          )                    R      where N represents the proportion of diode contacting areas between two diodes (D1, D2); their output voltages Vout can be represented as:       V    out    =            V      diode        +                  R1        R            ×              KT        q            ×              ln        ⁡                  (          N          )                    where Vdiode represents the voltage value across two ends of diode (D3).
In this model, the output voltage tends to decrease as the temperature rises, but the constant KT/q will increase along with the temperature. Using their complementing characteristics, a suitable reference voltage V0 can be obtained when the temperature is equal to T0.
On the other hand, when the ratio between two resistors (R1/R) is changed, the output voltage Vout will also be changed, which means the temperature coefficient can be used to control the output voltage Vout.
in, With reference to FIG. 9, the following example employs four bandgap reference circuits to generate output voltages with different temperature coefficient (TC1˜TC4). A multiplexer is used to select a particular bandgap reference circuit based on the actual temperature requirement. The output voltage is temperature compensated, and the output voltage is always equal to the reference voltage (V0) at a reference temperature T0. The main disadvantage of using this technique is that these bandgap reference circuits occupy a large space in the integrated circuit and consume considerable power. Therefore, under the precondition that the output voltage has to be equal to the reference voltage (V0) at a reference temperature T0, the most difficult task is finding suitable components having different temperature coefficients to produce different output voltages. This poses a challenge for fabrication of the present day semiconductor.
With reference to FIG. 10, another example of a bandgap reference circuit is based on the design described in the previous example, but the two bandgap reference circuits are connected in parallel in this example respectively with output voltages V1 and V2. To generate voltage values having different temperature coefficients, two diodes (D1, D2) are used, which are built with somewhat different processes to produce the necessary temperature characteristics. Unfortunately, this modulation process cannot be implemented in actual chip fabrication.
With reference to FIG. 11, another conventional bandgap reference circuit uses a single bandgap reference circuit to generate four different output voltages (V1˜V4) having different temperature coefficients. However, the output voltages (V1˜V4) are not equal at temperature T0, thereby four level conversion circuits are required for tuning the output voltages (V1˜V4) to make the voltage values (VTC1˜VTC4) converge at temperature T0. After the conversion by the level conversion circuit, the output voltages (V1˜V4) can then be defined as follows:VTCn=an×Vn+bn, where n=14. 
With reference to FIG. 12, still another bandgap reference circuit, is largely based on the previous design, but only two voltage outputs (V1,V2) are generated instead of four. These two output voltages are obtained from two resistors (R1,R2) connected in series. Using the same design logic, four resistors must be connected in series if four output voltages are needed.
The biggest drawback in the above design is that the level conversion circuit will complicate the circuit design, which not only takes up more space in circuit design but also uses more power. Furthermore, the temperature coefficient of each output voltage after the level conversion is also likely to be changed.